3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) technology is a mobile broadband wireless communication technology in which transmissions from base stations (referred to as eNBs) to mobile stations (referred to as user equipment (UE)) are sent using orthogonal frequency division multiplexing (OFDM). OFDM splits the signal into multiple parallel sub-carriers in frequency.
In order to demodulate any transmissions on the downlink, a UE relies on reference symbols (RS) that are transmitted on the downlink. These reference symbols and their position in the time-frequency grid are known to the UE and hence can be used to determine channel estimates by measuring the effect of the radio channel on these symbols. The common reference symbols are used for channel estimation during demodulation of control and data messages in addition to synchronisation.
The network architecture for LTE allows messages to be sent between eNBs via an X2 interface. The eNB also can communicate with other nodes in the network, e.g., to the Mobility Management Entity (MME) via the S1 interface.
In current specification methods are specified that allow some self-organizing network (SON) functionality where an eNB can request information regarding another eNB via the MME. Currently, network interface based signaling for over the air synchronisation purposes is enabled by means of the S1. It shall be noted that the Radio Interface Based Synchronisation (RIBS) functions standardised in 3GPP Release 12 have the purpose of enabling a more accurate detection of the synchronisation source signal, so to improve the synchronisation accuracy. Moreover, muting patterns activation should enable an enhancement of the synchronisation source signal with respect to the case where interference from aggressor cells is not mitigated.
It should be further noted that according to the 3GPP standard, the time offset between two adjacent eNBs shall be within 3 μs. The interpretation of the 3 μs requirement forces in practice a need of Global Navigation Satellite System (GNSS) receivers or IEEE1588v2 PTP, but GNSS cannot be deployed in some installation like for example indoor or in some urban environments and IEEE1588v2 PTP would require investment in the network.
The 3GPP standard does not define how RIBS or a chain of RIBS links should be configured. In most cases there would be several possible sync paths to consider and characteristics for such paths are likely to change over time by e.g. deployment of new nodes or changes in nearby infrastructure and environments.